Microstructural refinement of ?-sintered and Ti-6Al-4V porous-coated by temporary alloying with hydrogen

Abstract

A series of thermochemical treatments, in which hydrogen was used as a temporary alloying element to refine the lamellar microstructure of β-sintered and porous-coated Ti-6Al-4V was formulated. Each step of the treatment sequence (hydrogenation, eutectoid decomposition and dehydrogenation) was studied separately, on uncoated specimens and then on porous-coated specimens. The resultant microstructures can have α-grain sizes less than 1 μm, aspect ratios near unity and discontinuous grain boundary α (GBα), microstructural attributes which increase the fatigue strength. Microstructural refinement occurs because hydrogen-alloying reduces the (α+β)↔β transition temperature and enables a eutectoid decomposition reaction to occur. The optimal hydrogenation temperature is 850 °C, because hydrogen concentrations of 0.71 to 0.85 wt% are in-diffused and β-transformation is achieved. These weight percentages are in the optimal range for efficient eutectoid decomposition kinetics, β-transformation obviates the need for a separate β-transformation treatment step. A separate eutectoid decomposition treatment step may be used, or eutectoid decomposition may be combined with dehydrogenation. The finest eutectoid microstructures are obtained if hydrogen concentrations are in the range 0.5 to 0.8 wt%. The criteria for dehydrogenation are efficient removal of hydrogen, with minimal grain growth and absence of GBα. These criteria are best met by using dehydrogenation temperatures <700 °C. Altering the sintering temperature or adding a porous coating does not affect the parameters of the hydrogen-alloying treatment steps.